Novel sulfonated poly(arylene ether sulfone) copolymers containing intermolecular ionic cross-linkable moieties, 5,5'-bis[2,4-(hydroxyphenyl) benzimidazole] (BHPB), are synthesized by direct polymerization for application in polymer electrolyte membranes in DMFCs. The synthesized copolymers are controlled by various monomer compositions to confirm the effects of ionic cross-linking from sulfone-amine groups and degree of sulfonation on the polymer electrolyte membrane. The mechanical and thermal properties are enhanced in the ionic cross-linked membranes compared to the non-ionic cross-linked membrane. The water uptake and methanol permeability descend with decreasing degree of sulfonation and increasing content of BHPB. Similarly, regarding proton conductivity and ion exchange capacity (IEC), the low sulfonated membrane with high content of BHPB displays low values. This is attributed to the formation of ionic bonding by the sulfonic acid and amine groups. The novel polymer in this study shows very promising performance in direct methanol fuel cells which is comparable to that using Nafion 115. (C) 2011 Elsevier B.V. All rights reserved.